The present invention generally relates to a recording/reproducing apparatus of the rotary head type. More particularly, the invention relates to a rotary head recording/reproducing apparatus in which an audio signal is converted into a PCM (pulse code modulation) signal, and the PCM signal is recorded in the form of one inclined track for every predefined unit of time on a recording medium by means of a rotary head.
To record and reproduce an audio signal in the form of parallel inclined tracks on a magnetic tape by means of a rotary head of the helical scanning type, a so-called R-DAT (rotary-head digital audio tape recorder) has been proposed in which the input audio signal is converted into a PCM signal for recording/reproduction.
A track in which an audio signal is recorded with an R-DAT has a pattern as shown in FIG. 1A, in which each of the MARGIN, PLL, and POSTAMBLE regions has a frequency of 1/2 f.sub.M (f.sub.M =9.4 MHz) and an IBG region has a frequency of 1/6 f.sub.M. Each of the SUB and PCM regions is constituted by a block as shown in FIG. 1B. A SYNC region is constituted by 10 bits (nine bits being fixed), and the remaining regions have various patterns depending on locations and audio signals. Those blocks are repeated eight times and 128 times in the case of the SUB region and the PCM region, respectively. In the diagram of FIG. 1A, the numerical values represent the number of blocks constituting each of the regions.
An ATF-1 region (ATF: automatic track finding) disposed between the SUB-1 region and the PCM reigon and an ATF-2 region disposed between the PCM region and the SUB-2 region are provided for purposes of tracking control so that, during reproduction, the rotary head can accurately scan a recorded track on the basis of the output of the rotary head alone. Thus, there is no need to provide a special tracking control head.
That is, the ATF regions are used in controlling the tracking of the rotary heads during reproduction in the case where a PCM signal is time-compressed and recorded in the form of parallel inclined tracks with no guard band. Tracking pilot signals are recorded at the initiation and termination positions of each of the tracks in recording regions formed independently of the recording region in which the PCM signal is recorded. During reproduction, the recording track is scanned by a rotary head having a scanning width larger than that of the recorded track, and the tracking of the rotary head is controlled in response to the reproduced outputs of the pilot signals from the two side tracks adjacent to the track being scanned.
FIG. 2 shows the track patterns of the pilot signals recorded in those ATF regions. A description will be given as to the illustrated patterns in the case where the scanning head rotary drum has a diameter of 30 mm, a winding angle of 90 degrees, and a rotational speed of 2000 rpm.
In each of the ATF-1 and ATF-2 regions respectively disposed in the initiation and termination positions of each of the tracks, a signal f.sub.1 of a low frequency and having a small azimuth loss is recorded as a pilot signal for tracking control, and the signal fl is utilized during reproduction to detect the levels of crosstalk components from both the adjacent tracks. The difference in level between the crosstalk components is used as the tracking error signal. For the foregoing pilot signal f.sub.1, a low frequency signal of 1/72 f.sub.M (130 KHz) may be used.
Further, a synchronizing signal is recorded in each of the ATF-1 region and the ATF-2 region so as to discriminate the position where the pilot signal f.sub.1 is recorded. For the synchronizing signal, a signal of a frequency having an azimuth effect and of a pattern different from that of the PCM signal is selected because the center track (the track being scanned) cannot be discriminated from the two adjacent tracks when crosstalk is generated from the adjacent tracks. Assuming that a head A corresponds to plus azimuth and a head B corresponds to minus azimuth, the synchronizing signals are made to be different in frequency from each other so as to discriminate the head A and the head B from each other. For this purpose, a first synchronizing signal f.sub.2 of a frequency of 1/18 f.sub.M (=522 KHz) corresponding to the head A and a second synchronizing signal f.sub.3 of a frequency of 1/12 f.sub.M (=784 KHz) corresponding to the head B are recorded in predetermined positions.
In the R-DAT, no erase head is used, and signal rewriting is performed through an overwriting operation in which the new signal is written over the old signal. To this end, erase signals f.sub.4 each having a frequency of 1/6 f.sub.M (=1.56 MHz) are recorded in predetermined positions so as to erase the previously recorded pilot signal f.sub.1, first synchronizing signal f.sub.2, and second synchronizing signal f.sub.3.
The positions of all the pilot signals recorded in the respective ATF regions on the center track and its adjacent tracks are made different from each other, and the levels of the pilot signals in the center track and the adjacent tracks are made to differ in time so that three types of levels of the pilot signals can be sampled.
Each of the ATF-1 region and the ATF-2 region is constituted by five blocks, and the pilot signal f.sub.1 is recorded in two of the five blocks. The synchronizing signal f.sub.2 or f.sub.3 is recorded so as to occupy either one block or one-half block from the center of the position where one of the adjacent tracks is recorded. The pilot signal f.sub.1 on the other adjacent track is recorded such that the center of the pilot signal f.sub.1 is located at a position two blocks behind the head of the synchronizing signal recorded on the center track. The synhronizing signal of one block and the synchronizing signal of one-half block are assigned to an odd number frame track and an even number frame track, respectively.
As described above, in the ATF region, the frequency of the synchronizing signal corresponding to the head A is different from that of the synchronizing signal corresponding to the head B, and further the recording length of the synchronizing signal recorded on the odd number frame track is different from that of the synchronizing signal recorded on the even number frame track. Therefore, the ATF regions provided on four successive tracks are made entirely different in arrangement from each other so that four successive tracks can be discriminated from each other. Thus, ATF pattern described above is repeated every four tracks; that is, the ATF pattern is of the four-track completion type.
When a magnetic tape carrying an audio signal recorded in the format shown in FIG. 1A is reproduced by a rotary head, an RF signal as shown in FIGS. 3A-3C is obtained from the rotary head. In the case where the RF signal is obtained, for example, by the reproduction of an odd number frame track (A) shown in FIG. 2, the RF signal is passed through a BPF (bandpass filter) of 130 KHz to thereby obtain a pilot signal f.sub.1 as shown in FIG. 3B.
In FIG. 3B, the section I is obtained from the pilot signal of the center track, and sections II and III are formed from crosstalk of the pilot signals of an odd number frame track (B) and an even number frame track (B), respectively. If the center track is accurately scanned by the rotary head, the envelope levels of the sections II and III, that is, the amplitudes V.sub.II and V.sub.III indicated in FIG. 3C, will be equal to each other. However, if there is a track displacement, the amplitudes V.sub.II and V.sub.III will be different from each other, and the amplitude and direction of displacement of the rotary head relative to the center track can be detected by the amplitude and polarity of the difference between the amplitudes V.sub.II and V.sub.III. Therefore, the rotary head can be made to accurately follow the center track if the capstan servo controlling the speed of the tape is driven in accordance with the difference between the amplitudes V.sub.II and V.sub.III so as to finely adjust the tape speed.
In order to carry out the foregoing operation, it is necessary to accurately detect the synchronizing signals recorded in the predetermined positions so as to sample the levels V.sub.II and V.sub.III. However, as described above, the R-DAT is provided with no erase head, and second, third, etc., recording operations are carried out by overwriting. Accordingly, in the conventional R-DAT, there has been the possibility that the synchronizing signals cannot be accurately detected in sampling the levels V.sub.II and V.sub.III so that a proper error signal cannot be generated.
That is, recording may be made within .+-.2 blocks from the center of the PCM region, and the pilot signal f.sub.1 (=130 KHz) recorded at a level which is slightly lower than those of other signals. This is because a signal of a lower frequency is recorded physically more deeply into the tape. However, the previously recorded pilot signal f.sub.1 must be erased by a higher frequency erase signal. If the level of the pilot signal f.sub.1 is lowered, there is thus the possibility that when the pilot signal f.sub.1 is newly recorded over a previously recorded synchronizing signal f.sub.2 or f.sub.3, the previously recorded synchronizing signal cannot be entirely erased.
For example, in the case where the succeeding (new) recording is displaced backwardly, the synchronizing signal remaining after erasure will be advanced with respect to the newly recorded synchronizing signal. In a typical example of such a case, the pilot signal f.sub.1 is displaced backwardly within 1-2 blocks so that a part or all of the previously recorded synchronizing signal f.sub.2 or f.sub.3 remains after erasure at a region where the pilot signal f.sub.1 is to be recorded, namely, in the odd number frame (A) or in the even number frame (A) in the case of the ATF-1 region, or in the odd number frame (B) or in the even number frame (B) in the case of the ATF-2 region.
When such displacement occurs, the level of the frequency component of the pilot signal of the reproduced RF signal at that time is sampled which corresponds to the previously recorded synchronizing signal. The level of this pilot signal should be the level of crosstalk of the sampling signal of one of the adjacent tracks. However, the foregoing sampled frequency component is the pilot signal on the center track, and hence the level obtained by sampling has an exceedingly large value. Thereafter, the frequency component of the pilot signal in the reproduced RF signal after two blocks is sampled, the difference in level between this sampled value and the sampled value two blocks earlier is obtained, and the capstan servo is controlled in response to the level difference, which is taken as indicative of the amount of the track displacement. The previously sampled value, however, is not the crosstalk level of the adjacent track, but the level of the center track, and therefore, as a level difference, an extremely large value greatly different from that accurately indicative of the actual track displacement is obtained. As a result, the capstan servo cannot operate properly.
Although usually no problem arises in the case where a previously recorded pilot signal f.sub.1 is erased by a new pilot signal f.sub.1 using the same apparatus, it is a matter of course that there are variations in recording levels among different apparatuses, especially among apparatuses produced by different makers. Hence there will unavoidably occur such a problem that, for example, a signal is recorded deeply in recording level in an apparatus A while shallowly in another apparatus B. In such a case, there is still no problem if the overwriting operation on a tape recorded by the apparatus B is carried out with the apparatus A, but the problem will obviously occur when a previously recorded pilot signal f.sub.1 cannot be erased by a new pilot signal when the overwriting operation on a tape recorded by the apparatus A is carried out with the apparatus B. In this case, the previously recorded pilot signal and the newly overwritten pilot signal interfere with each other such that they are added to or subtracted from each other. Therefore, even when a synchronizing signal is accurately detected, there occurs a problem that the amount of track displacement cannot be accurately detected because of the interference between the pilot signals f.sub.1. In the worst case, the center of the DCM region will not be displaced with the same azimuth.
In the R-DAT, there have been the foregoing various problems because no erase head is provided and erasure is performed only by overwriting operations. In order to solve the foregoing problems, an erase head may be provided which contacts the tape and applies a magnetic field to the tape to thereby erase the previously recorded signals. Otherwise a "flying" erase head may be used, similarly to the conventional analog tape recorder.
In the former case of using an erase head, however, it is necessary to separate the erase head from the tape during reproduction. Therefore, the tape path in the reproducing mode is different from that in the recording mode, making it more likely that tape running problems will occur. Further, in the latter case of using a flying erase head, although the above-mentioned problem is eliminated, since it is necessary to attach the erase head to a rotary drum having a very small diameter of not larger than 30 mm, there occur manufacturing problems and higher costs are inevitable.